1. Field of the Invention
This invention relates to a carboxyl group-containing photosensitive resin obtained by addition reaction of a phenolic resin having a particular structure with a cyclic ether group-containing compound, such as an alkylene oxide and a cyclic carbonate, and the subsequent addition of an α,β-ethylenically unsaturated group-containing monocarboxylic acid and the addition of a polybasic acid anhydride thereto, of which cured product excels in resistance to heat and toughness, exhibits high hardness and flexibility, and enjoys high water resistance and resistance to chemicals.
This invention further relates to an alkali-developable and curable resin composition containing such a carboxyl group-containing photosensitive resin, a dry film thereof, and a printed circuit board containing a cured product obtained therefrom. More particularly, this invention relates to a liquid alkali-developable, photocurable and thermosetting resin composition which is suitable for use as a permanent mask to be used in the manufacture of usual printed circuit boards, flexible printed circuit boards, and tape carrier packages, interlaminar insulating layers for multi-layer circuit boards, and the like and capable of forming a cured film excelling in adhesiveness, resistance to soldering heat, resistance to moisture absorption, resistance to PCT (pressure cooker test), resistance to electroless gold plating, flexing resistance, folding endurance, flexibility, warpage, and electrical insulating properties by means of a procedure comprising the steps of irradiating a coating film of the composition with ultraviolet light, developing the exposed film with a dilute aqueous alkali solution, thereby forming an image on the film, and finally curing the film by heating, or by irradiation with active or actinic energy rays and subsequent heating, or by heating and subsequent irradiation with active energy rays.
2. Description of the Prior Art
At present, as a solder resist for part of the household grade printed circuit boards and for virtually all the industrial grade printed circuit boards, a liquid developing type solder resist which is irradiated with ultraviolet light and then developed to form an image and thereafter finally cured (finish curing) by heating or exposure to light is adopted from the viewpoint of ensuring highly accurate formation of circuits with high density. Further, with due respect to the problem of environmental safety, the liquid solder resist of the alkali developing type which implements development with a dilute aqueous alkali solution as a developer has come to play the leading role. As such alkali developing type solder resists using a dilute aqueous alkali solution, for example, published Japanese Patent Application, JP 61-243869A discloses a solder resist composition comprising a photosensitive resin obtained by addition of an acid anhydride to a reaction product of a novolak type epoxy compound with an unsaturated monobasic acid, a photopolymerization initiator, a diluent, and an epoxy compound, JP 3-253093A discloses a solder resist composition comprising a photosensitive resin obtained by addition of an acid anhydride to a reaction product of a novolak type epoxy compound with an unsaturated monobasic acid, a photopolymerization initiator, a diluent, a vinyltriazine or a mixture of vinyltriazine with dicyandiamide, and a melamine resin, and JP 3-71137A and JP 3-250012A disclose a solder resist composition comprising a photosensitive resin obtained by addition of (meth)acrylic acid to an epoxy resin resulting from the reaction of epichlorohydrin with a reaction product of salicylaldehyde and monovalent phenol and subsequent addition of a polybasic carboxylic acid or anhydride thereof to the resultant resin, a photopolymerization initiator, an organic solvent, etc.
As mentioned above, some material systems are heretofore proposed as a solder resist and currently used in large quantities in the practical manufacture of printed circuit boards. However, in view of high densification of the printed circuit board to cope with the recent trend of electronic devices toward decreasing weight and size, the desirability of producing the solder resist having high performance has been finding growing recognition. Furthermore, the IC packages using a printed circuit board having the solder resist applied thereto and a sealing resin are recently used instead of the IC packages called QFP (quad flat-pack package), SOP (small outline package), etc. which use a lead frame and a sealing resin. These new packages have such structure that metals such as ball-like solder are arranged in an area on one side of a printed circuit board having the solder resist applied thereto, the IC chips are directly connected thereto on the other side by wire bonding or through the medium of the bumps etc., and they are sealed by a sealing resin. They are called by mnemonic names, such as BGA (ball grid array) and CSP (chip scale package). These packages have more pins than other packages such as QFP of the same size, but are easier to miniaturize. Moreover, also in mounting the devices, they realize a low percent defective owing to the self-alignment effect of ball-like solder, and the introduction of these packages is advanced quickly.
In the printed circuit boards having the heretofore commercially available alkali development type solder resist, however, the PCT resistance which is the long-term reliability test of a package is inferior, which results in exfoliation of a cured film. Moreover, another matter which comes into question is the so-called popcorn phenomenon, i.e. the phenomenon that the moisture absorbed in the solder resist boils within the package during re-flow of the package at the step of mounting of devices and cracks occur in the solder resist film and the circumference thereof inside the package. Such problems in resistance to moisture absorption and long-term reliability are undesirable not only in the case of the above-mentioned mounting technology but also in the products for other uses, such as a solder resist of a usual printed circuit board, a solder resist to be used in the manufacture of a flexible printed circuit board and a tape carrier package, and an interlaminar insulating layer of a multi-layer circuit board like a build-up board.
As mentioned above, the recent advance of the electric industry and semiconductor industry requires more improvement in the characteristics such as, for example, heat resistance, toughness, flexibility, water resistance, and resistance to chemicals. In order to satisfy such demands, various new photosensitive resins are developed.
Heretofore, the photosensitive resin originating from the novolak type epoxy resin as a starting material is widely used in many fields of electronic material, such as a solder resist and an etching resist, owing to its outstanding adhesiveness, heat resistance, resistance to chemicals, electric insulation, etc. As a carboxyl group-containing photosensitive resin having particularly outstanding heat resistance, the above-mentioned resin obtained by causing a polybasic acid anhydride to react with a reaction product of a cresol novolak type epoxy resin and an unsaturated group-containing monocarboxylic acid (JP 61-243869A) is widely used. This resin is excellent in heat resistance. However, this resin has the drawback of being easy to generate cracks by the thermal shock because it causes large shrinkage during curing and exhibits low elongation and poor toughness.
As photosensitive resins which will solve such problems, a photosensitive prepolymer which is a reaction product of a bisphenol type epoxy resin having side chains of which hydroxyl groups is partially epoxidized, a (meth)acrylic acid, and a polybasic acid anhydride (JP 9-54434A), an unsaturated group-containing polycarboxylic resin obtained by causing a tetrahydrophthalic anhydride to react with a reaction product of a cresol novolak type epoxy resin, acrylic acid, and p-hydroxyphenethyl alcohol (JP 11-288091A), etc. are proposed. However, these resins are also still insufficient to satisfy both the heat resistance and toughness.
Furthermore, there is proposed a carboxyl group-containing photosensitive resin obtained by modifying a novolak resin with an alkylene oxide and then adding a (meth)acrylic acid and further a polybasic acid anhydride to the resultant reaction product (Japanese patent No. 3964326). Although this resin exhibits excellent long-term reliability, there is much room for improvement in drying characteristics of ink after application thereof.